The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells as a specific example of the prior art.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the backside. It is well-known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts that are electrically conductive.
Most electric power-generating solar cells currently used on earth are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing to form a metal paste.
An example of this method of production is described below in conjunction with FIG. 1. FIG. 1 shows a p-type silicon substrate, 10.
In FIG. 1(b), an n-type diffusion layer, 20, of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the phosphorus diffusion source. In the absence of any particular modification, the diffusion layer, 20, is formed over the entire surface of the silicon substrate, 10. This diffusion layer has a sheet resistivity on the order of several tens of ohms per square (Ω/□), and a thickness of about 0.3 to 0.5 μm.
After protecting one surface of this diffusion layer with a resist or the like, as shown in FIG. 1(c), the diffusion layer, 20, is removed from most surfaces by etching so that it remains only on one main surface. The resist is then removed using an organic solvent or the like.
Next, a silicon nitride film, 30, is formed as an anti-reflection coating on the n-type diffusion layer, 20, to a thickness of about 700 to 900 Å in the manner shown in FIG. 1(d) by a process such as plasma chemical vapor deposition (CVD).
As shown in FIG. 1(e), a silver paste, 500, for the front electrode is screen printed then dried over the silicon nitride film, 30. In addition, a backside silver or silver/aluminum paste, 70, and an aluminum paste, 60, are then screen printed and successively dried on the backside of the substrate. Firing is then typically carried out in an infrared furnace at a temperature range of approximately 700 to 975° C. for a period of from several minutes to several tens of minutes.
Consequently, as shown in FIG. 1(f), aluminum diffuses from the aluminum paste into the silicon substrate, 10, as a dopant during firing, forming a p+ layer, 40, containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
The aluminum paste is transformed by firing from a dried state, 60, to an aluminum back electrode, 61. The backside silver or silver/aluminum paste, 70, is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer, 40. Because soldering to an aluminum electrode is impossible, a silver back electrode is formed over portions of the back side as an electrode for interconnecting solar cells by means of copper ribbon or the like. In addition, the front electrode-forming silver paste, 500, sinters and penetrates through the silicon nitride film, 30, during firing, and is thereby able to electrically contact the n-type layer, 20. This type of process is generally called “fire through.” This fired through state is apparent in layer 501 of FIG. 1(f).
In the formation of the backside electrodes, there are two prior art methods which are typically used to form the Aluminum and silver electrodes (or Ag/Al electrode). The first method is disclosed in Japanese Kokai Patent Application Nos. 2001-127317 and 2004-146521 wherein a Ag paste is printed and dried, an Al paste is printed and dried, and both pastes are baked (to form the silver and Al electrodes).
The second method, as disclosed in Japanese Kokai Patent Application Hei 11[1999]-330512, is a process in which the formation sequence of the Al electrode and the Ag electrode of the first method is reversed wherein the paste for the Al electrode is printed and dried first, the Ag paste is printed and dried second, and both pastes are baked.
In the second method, cracks were apt to be generated in the superposed part of two electrodes due to the difference in the thermal contraction behavior of the Al electrode and the Ag electrode, so that the electrical characteristics (conversion efficiency, etc.) of the solar cell were degraded.
Japanese Kokai Patent Application No. 2003-223813 considers the thermal contraction of aluminum. In this publication, it is presented that the thermal contraction of Al is improved by including a material with a low thermal expansion coefficient, such as SiO2 in the paste composition. However, in this publication, although the decrease of the characteristics of the solar cell is improved by the addition of SiO2, it is not described that cracks of the superposed part of the Al electrode and the Ag electrode do not occur by the addition of SiO2.
Additionally, when SiO2 is used, there is a possibility that the solderability will be lowered, and the suppression of the thermal contraction and the solderability have a trade-off relationship. Usually, since the material of the second electrode used as an electrode for connection requires adhesive strength, as well as solderability, it must be designed so that glass particles used as a binder are necessarily included in the material to improve the adhesion with the Si substrate.
In the conventional electroconductive pastes, it was difficult to obtain an electroconductive paste for connection that suppresses the generation of cracks and that can form the second electrode having both a sufficient adhesive strength and that can maintain sufficient cell characteristics.
The present invention solves the above-mentioned problems by providing an electroconductive paste that essentially does not generate cracks in the superposed part, even by simultaneously baking an aluminum electrode and a silver electrode of the back face. This invention can form an electrode with a sufficient adhesive strength while still maintaining sufficient solar cell characteristics.